StMoMo v0.4.2 -- Fix dependency issue to forecast and gnm packages + forecast.mrwd() fix

[mpascariu]

News:

• create the forecast method to be used by StMoMo and mrwd objects. And also make sure that the models from forecast package (arima, etc) are using their own method. This solves the forecast.mrwd() problem. Examples included.
• having a forecast method in the package, the forecast dependency is no longer required. Now we just import the necessary function from forecast.
• The use of Mult() in StMoMo is somehow tricky, and as it is implemented in the package, this can not be an internal function. Now I understand with it was necessary to have gnm loaded prior to StMoMo. However, a workaround is to define our own Mult() and export it from StMoMo. In this way all we have to do is to import gnm::gnm().

[mpascariu]

Creating forecast method in StMoMo solves forecast.mrwd() issue; BUT it may create problems if forecast and StMoMo packages are loaded in the same time. Maybe this is not the best solution...